Method and installation for separating and purifying diphenols in the phenol and phenol derivatives industry

ABSTRACT

A method and installation for separating and purifying a crude mixture containing hydroquinone, resorcinol and possibly tars and/or catechol, comprising the following steps: —a possible distillation stage (I) in order to obtain a catechol head, —the foot (I) or crude mixture undergoes distillation (II) in order to obtain a fraction that is rich in resorcinol, —the foot of (II) undergoes distillation (III) in order to obtain a fraction that is rich in hydroquinone, whereupon said rich fractions are refined (IV or V). Preferably, one or several stages in which tar is removed (I,I′) precede stage (I) or (II).

The present invention relates to a process for the separation andpurification of crude mixtures essentially comprising hydroquinone andresorcinol, optionally tars and optionally catechol, in order to extracttherefrom first the hydroquinone and secondly the resorcinol, and thecatechol, when it is present, and optionally to purify these variouscompounds. It also relates to the plants which allow this process to beimplemented.

The phenol and phenol derivatives industry generates large volumes ofbyproducts comprising, among a great variety of tars, the para, orthoand meta derivatives of dihydroxybenzene. They are hydrocuinone (paracompound: 1,4-dihydroxybenzene), catechol or pyrocatechin (orthocompound: 1,2-dihydroxybenzene) and resorcinol or resorcin (metacompound: 1,3-dihydroxybenzene).

These three compounds have an added value but their extraction from suchcomplex mixtures is not without presenting problems of a technicalnature and an economic nature. Moreover, hydroquinone and resorcinol areisomers which are particularly difficult to separate.

FR-A-2 467 185 discloses a process for the separation and purificationof resorcinol and hydroquinone involving stages of distillation and ofrecrystallization by using a solvent such as water or an organicsolvent. According to one alternative form, this process providesdistillation stages using steam for entraining the hydroquinone in theform of hydroquinone vapor. This process uses a third solvent whichsubsequently has to be removed, which requires additional stages anddevices, for example for filtration and for drying, and optionally forreprocessing or recycling the solvent.

An object of the present invention, which relates in particular to theseparation and the purification of diphenols in the phenol and phenolderivatives industry, is to provide an appropriate method and plantwhich make it possible to separate and to purify, under favorableeconomical conditions, hydroquinone and resorcinol from a crude mixture.

Another object of the invention is to make possible the separation andthe purification of first hydroquinone and secondly resorcinol from acrude mixture comprising other compounds, in particular tars, and/oroptionally catechol, and also to separate and purify the catecholoptionally present.

Another object of the invention is to provide such a process which canbe operated largely continuously.

Yet another object of the invention is to provide such a process andplant which make it possible to obtain hydroquinone, resorcinol andcatechol having a high purity, in particular of greater than 98%,preferably than 99%, indeed even greater than or equal to 99.5%.

Yet another object of the invention is to provide such a process whichdoes not require the use of a third solvent.

These objects are achieved in accordance with the invention by a processfor the purification of a crude mixture comprising hydroquinone andresorcinol, optionally tars, and optionally catechol, in which processthe crude mixture is subjected to a series of distillation stages,preferably carried out continuously, comprising:

-   -   (i) an optional first distillation stage (I) designed to produce        catechol as distillation top product; this stage is carried out        when the crude mixture comprises catechol, in particular when        the content of catechol in the crude mixture exceeds 2%        inclusive,    -   (ii) the distillation bottom product obtained under (i) where        the crude mixture in the absence of stage (I) is subjected to a        distillation stage (II) designed to produce, as distillation top        product, a resorcinol-rich fraction comprising resorcinol,        essentially, and hydroquinone,    -   (iii) the distillation bottom product obtained under (ii) is        subjected to a distillation stage (III) designed to produce, as        distillation top product, a hydroquinone-rich fraction        comprising hydroquinone, essentially, and resorcinol,        and then the hydroquinone-rich fraction and/or the        resorcinol-rich fraction is/are subjected to a refining stage        (IV, V) in order to extract the hydroquinone and/or the        resorcinol.

In order to improve the yield for the recovery of hydroquinone from thecrude mixture, it is preferable to precede stages (I) and/or (II) by atleast one predistillation “detarring” stage (1) which makes it possibleto remove the tars as distillation bottom product. It is even preferablethen to redistil this distillation bottom product in at least one secondpreliminary detarring stage (1′) and to recover the distillation topproduct, capable of comprising a certain amount of the desiredcompounds. The distillation [lacuna] or the two (or more) distillationtop products thus obtained are conveyed as feed mixture to stage (I), ifsuch a stage is provided, or stage (II), in the contrary case. Morepreferably, these preliminary stages are carried out continuously withthe distillation stages which follow.

The mixtures to which the process applies are mainly those comprising inparticular, with respect to the total mixture:

-   -   from 20 to 60%, in particular from 30 to 50%, by weight of        hydroquinone,    -   from 2 to 20%, in particular from 2 to 15%, by weight of        resorcinol,    -   from 0 to 20%, in particular from 5 to 15%, by weight of        catechol,    -   the remainder being formed of various compounds, essentially        tars.

The “detarring” distillation stages (1, 1′) can be carried out withscraped falling film devices of conventional design or short pathdevices. However, the use of multistage columns is not ruled out (see,e.g., column (III)). The aim is simply to remove as much as possible ofthe tars without a significant loss of the desired compounds.

If stages (1 and 1′) are not provided, it is preferable to use columns(I) and (II) with antifouling packings in order to limit the foulingthereof by the tars. Such packings are fully known to a person skilledin the art.

Stage (I) is targeted simply at extracting the catechol and thus atobtaining, as top product, catechol with a purity which is as high aspossible. The aim in particular is to obtain a fraction comprising atleast 98%, preferably at least 99%, of catechol.

The term “rich” as used above for stages (II) and (III) is understood tomean that the compound targeted is the major component, the othercompound being a minor component but present in a sufficient amount tosubsequently make possible the refining. A person skilled in the art isentirely in a position to determine by routine tests the ranges ofratios, basing himself on the crystallization curve of aresorcinol/hydroquinone mixture, in order to determine the ratioscorresponding to the range of the eutectics. From this information, byvarying the operating parameters of the columns, it is possible toachieve conditions such that the rich fractions have a ratio whichappears on either side of this range, as is known per se, which willallow the subsequent implementation of the refining.

The operating conditions of stages (II) and (III) are thus related. Eachis targeted at the production, as distillation top product (as columntop product), of a hydroquinone/resorcinol mixture which is compatiblewith the subsequent refining stage.

It is thus preferable for stage (III) to result in a mixture comprising:

-   -   from 75 to 95%, preferably from 85 to 92%, of resorcinol,    -   from 5 to 25%, preferably from 8 to 15%, of hydroquinone.        (Possible residues of other compounds, e.g. catechol, which        remain minor components, are not taken into account).

These operating conditions make it possible to ensure, during stage(III), the production as distillation top product of a mixturecomprising in particular:

-   -   from 75 to 98%, preferably from 85 to 97.5%, of hydroquinone,    -   from 2 to 25%, preferably from 2.5 to 15%, of resorcinol.    -   (Here again, possible residues of other compounds which may be        present in negligible amounts are not taken into account).

From this information, a person skilled in the art is fully in aposition to choose the means to be employed according to the startingmixture. The following should simply be noted. The size (in particularthe diameter) of the distillation columns depends on the circulatingstream and on the internal pressure. They will thus be dimensionedmainly according to the flow rate of the mixture to be treated. Theinternal parameter which is the number of theoretical stages isdetermined in particular by the composition (ratios) of the enteringmixture and the purity or the composition of the mixture which has to beobtained as distillation top product and as distillation bottom product.It will be specified that the columns may without distinction be packedwith plates or with stacked packing, as is fully known to a personskilled in the art. The plant having been determined, a person skilledin the art adjusts the operating parameters of the columns.

Thus, the distillation column (I) can advantageously but not limitinglybe a column having the following specifications:

-   -   number of theoretical stages: from 5 to 40, preferably from 10        to 30;    -   reflux ratio R of between 1 and 10, preferably between 2 and 5.

The distillation column (II) can advantageously but not limitingly be acolumn having the following specifications:

-   -   number of theoretical stages: from 10 to 85, preferably from 15        to 40,    -   reflux ratio R of between 1 and 35, preferably between 5 and 25.

The distillation column (III) can very simply be a column of type (1) oralternatively a column having the following specifications:

-   -   number of theoretical stages: from 1 to 10, preferably from 1 to        5,    -   reflux ratio R of between 0.5 and 5, preferably between 1 and 2.

The refining is carried out batchwise using devices which make possibleliquid/solid separation (draining, zone melting) and which aredimensioned according to the volume to be treated and their number. Thechoice of the type of device is not critical either. They can, forexample, be conventional drainers or other refining devices, for examplethose sold under the name Proapt (registered trademark). It is possible,for example, to use drainers of the type with a vertical cylindricaltubular exchanger.

The treatment of the rich fractions in these devices is carried outessentially according to the four following phases:

-   -   phase 1 corresponds to the slow crystallization of the charged        mixture    -   phase 2 corresponds to the cold draining of the eutectic        (resorcinol and hydroquinone mixture)    -   phase 3 corresponds to the hot draining recovered during the        reheating phase until the desired purity is obtained    -   phase 4 corresponds to the melting-recovery of the pure product.

The production of fractions with substantially constant compositionsalso makes it possible to automate the progress of this refining.

The resorcinol-rich fraction is conveyed to one or more refiningdevice(s). Before phase 1, the device is heated above the melting pointof resorcinol (11° C.), i.e., for example, between 115 and 120° C.

During phase 1, the body of material is cooled, e.g. to a temperature ofbetween 40 and 90° C., over several hours, e.g. over from 5 to 15 h,which results in the slow crystallization of the charged mixture.

After phase 1, the product which has remained liquid is withdrawn fromthe device (phase 2) before passing to phase 3.

Phase 3 consists of the slow reheating of the refining device,optionally begun during phase 2, e.g. up to a temperature of between 109and 111° C., over several hours, e.g. over from 8 to 15 h. The end ofphase 3, which conditions the purity of the product, can be determinedeither by measuring the crystallization point or by any otherphysiochemical analytical technique.

Phase 4 provides for heating of the device to a temperature greater than115° C., so as to melt the resorcinol, which is withdrawn in the moltenstate.

The hydroquinone-rich fractions are treated in the same way. Thetreatment follows the same phases, apart from the heating/coolingtemperatures and times. By way of example:

-   -   preheating between 175 and 180° C.    -   phase 1, cooling between 90 and 130° C.    -   phase 1, duration between 5 and 15 h    -   phase 3, heating between 170 and 173° C.    -   phase 3, duration between 8 and 24 h    -   phase 4, heating above 178° C.

The eutectic fractions recovered during the refining can be recycled asa mixture or separately with the hot drainings, preferably in stages(II) and/or (III). It is possible to be induced to recycle them in stage(I), if need be.

Another subject matter of the present invention is a plant which makespossible the implementation of the process described above, comprising:

-   -   (i) an optional distillation column (I) designed to produce        catechol at the column top,    -   (ii) a distillation column (II), the inlet of which is connected        to the bottom of column (I) or receives the crude mixture in the        absence of column (I), this column (II) being designed to        produce, at the column top, a resorcinol-rich fraction        comprising resorcinol, essentially, and hydroquinone,    -   (iii) a distillation column (III), the inlet of which is        connected to the bottom of column (II), this column (III) being        designed to produce, at the column top, a hydroquinone-rich        fraction comprising hydroquinone, essentially, and resorcinol,    -   (iv) one or more refining devices (IV, V) for providing for the        refining of the hydroquinone-rich fraction and/or the        resorcinol-rich fraction in order to extract hydroquinone and/or        resorcinol respectively.

In accordance with the preferred embodiment of the invention, this plantadditionally comprises:

-   -   a detarring column (1) designed to produce, at the column top, a        detarred fraction and, at the bottom of the column, a tar-rich        fraction    -   optionally at least one other distillation column (1′) fed with        the tar-rich fraction originating from the preceding column (1)        and designed to produce, at the column top, a detarred fraction        and, at the bottom, a tar-rich fraction, the top fraction or        fractions of these columns being used to feed column (I) or        (II).

The other information and characteristics given above with respect tothe process apply directly to the plant according to the invention.

The invention will now be described in more detail with the help ofembodiments taken as nonlimiting examples and with reference to thedrawing, in which:

FIG. 1 shows the diagram of a first plant in accordance with theinvention

FIG. 2 shows the diagram of a second plant in accordance with thepreferred embodiment of the invention.

EXAMPLE 1 (FIG. 1)

1st Distillation Column (I);

-   n (number of theoretical stages)=30-   R (reflux ratio)=2.7-   Column top temperature=176.4° C.-   Pressure=100 mmHg, i.e. 13 332 Pa.

This column (I) is fed continuously with a flow rate of 25.5 kg/h of amixture to be treated comprising:

-   -   approximately 50% hydroquinone, i.e. approximately 12.75 kg/h    -   approximately 15% catechol, i.e. approximately 3.8 kg/h    -   approximately 10% resorcinol, i.e. approximately 2.55 kg/h    -   approximately 25% tars, i.e. approximately 6.4 kg/h.

A flow rate of approximately 3.8 kg/h is obtained at the column top,which flow rate comprises:

-   -   approximately 99.5% catechol    -   approximately 800 ppm hydroquinone    -   approximately 40 ppm resorcinol.

A flow rate of approximately 21.7 kg/h is obtained at the column bottom,which flow rate comprises:

-   -   approximately 58.9% hydroquinone (approximately 12.75 kg/h)    -   approximately 11.7% resorcinol (approximately 2.55 kg/h)    -   approximately 180 ppm catechol    -   approximately 29.4% tars (approximately 6.4 kg/h).        2nd Distillation Column (II):

-   n=30

-   R=10

-   Column top temperature: 210° C.

-   Pressure: =100 mmHg, i.e. 13 332 Pa.

It is fed continuously with the bottom product from the 1st column at aflow rate of approximately 21.7 kg/h.

A flow rate of approximately 2.56 kg/h of a resorcinol-rich fraction isobtained at the top, which fraction comprises:

-   -   approximately 90% resorcinol (approximately 2.3 kg/h)    -   approximately 10% hydroquinone (approximately, 0.26 kg/h,    -   approximately 1 200 ppm catechol.

A flow rate of approximately 19.14 kg/h of a mixture is obtained at thebottom, which mixture comprises:

-   -   approximately 65.3% hydroquinone (approximately 12.49 kg/h)    -   approximately 1.3% resorcinol (approximately 0.25 kg/h)    -   approximately 33.4% tars (approximately 6.4 kg/h).        3rd (Distillation) Detarring Column (III):

-   Detarring column: scraped falling film device

-   Column top temperature: 217° C.

-   Pressure: 100 mmHg, i.e. 13 332 Pa.

This column is fed continuously with the bottom product from the 2ndcolumn at a flow rate of approximately 19.14 kg/h

A flow rate of approximately 9.64 kg/h of a hydroquinone-rich fractionis obtained at the top, which fraction comprises:

-   -   approximately 97.4% hydroquinone (approximately 9.39 kg/h)    -   approximately 2.6% resorcinol (approximately 0.25 kg/h)

A flow rate of approximately 9.5 kg/h of a mixture is obtained at thecolumn bottom, which mixture comprises:

-   -   approximately 32.6% hydroquinone (approximately 3.1 kg/h)    -   approximately 67.4% tars (approximately 6.4 kg/h).

The column bottom product can optionally be redistilled on a detarringcolumn.

Refining:

The refining makes it possible to obtain the pure products from the richfractions. Drainers of the type with a vertical cylindrical tubularexchanger were used. Similar results will be obtained with other typesof devices.

The operating method is as follows:

a) for the hydroquinone-rich fraction:

-   -   Charging: before the charging of approximately 180 kg of        hydroquinone-rich fractions, the drainer (V) is preheated to a        temperature greater than the melting point of hydroquinone, in        this instance to approximately 180° C.    -   Cooling: the body of material is slowly cooled by circulation of        water to a temperature of approximately 120° C. (cooling time        approximately 10 h).    -   Recovery of the eutectic fraction: the eutectic fraction, which        is also known as cold drainings, corresponds to the        uncrystallized part of the mixture at the end of cooling and is        a mixture of resorcinol and hydroquinone. In the case of these        drainers, this fraction can be recovered by simple gravimetric        flow and collected in a tank provided for this purpose. This        phase lasts approximately 12 hours and takes place with slow        reheating of the drainer.    -   The reheating of the drainer is continued in order to carry out        the hot draining phase. The end of the phase of recovery of the        hot drainings is determined by the measurement of the        crystallization point of the product which seeps out during this        heating phase. This fraction is recovered by simple gravimetric        flow and is collected in a tank provided for this purpose. This        fraction can either be recycled to the following refining        operation or mixed with the cold draining fraction and recycled        to the distillation.    -   Recovery of the pure hydroquinone: when the crystallization        point (171° C.) is reached, the flow of the hot drainings is        interrupted and the drainer is heated to a temperature of        180° C. in order to melt all the hydroquinone. Approximately 65        kg of hydroquinone are recovered with an assay of greater than        or equal to 99.5%.

b) For the resorcinol-rich fraction: the processing is carried out inthe same way as under a) with the drainer (IV), apart from the essentialdifference that this time it is the melting temperature of resorcinolwhich is taken into account, which temperature is 111° C. The heatingtemperatures are consequently modified.

-   Charging temperature 120° C.-   Cooling to 60° C. over approximately 10 h-   Recovery of the cold draining fraction over approximately 10 h-   Reheating from 60 to 110.5° C., the end of this reheating being    determined by the measurement of the crystallization point, which    determines the final purity of the product.-   Heating to 120° C. in order to recover the resorcinol: 65 kg with a    purity of greater than or equal to 99%.

EXAMPLE 2 (FIG. 2)

In comparison with example 1, two detarring columns (1 and 1′) are addedupstream of the distillation column (I) to remove at the start the tarspresent. The first (1) of these columns is fed with the mixture to betreated and the second (1′) with the bottom product from the precedingcolumn (1). The streams originating from the two column tops feed the1st column (I) according to example 1.

Detarring Columns

-   Scraped falling film devices-   Column top temperature: 174° C.-   Pressure: 10 mmHg, i.e. 1 333.2 Pa.

The column (1) is fed continuously with a flow rate of 35 kg/h with amixture to be treated comprising:

-   -   approximately 45% hydroquinone, i.e. approximately 15.75 kg/h    -   approximately 7% catechol, i.e. approximately 2.45 kg/h    -   approximately 3% resorcinol, i.e. approximately 1.05 kg/h    -   approximately 45% tars, i.e. approximately 15.75 kg/h.

The top products from the two detarring columns are combined and producea flow rate of approximately 18.9 kg/h of a detarred fractioncomprising:

-   -   approximately 2.45 kg/h catechol    -   approximately 15.3 kg/h hydroquinone    -   approximately 1.05 kg/h resorcinol    -   approximately 0.1 kg/h tars.

A flow rate of approximately 16.1 kg/h of a tar-rich fraction isobtained at the bottom of the column (1′), which fraction comprises:

-   -   approximately 15.65 kg/h tars    -   approximately 0.45 kg/h hydroquinone        Distillation column (I):

-   n (number of theoretical stages)=30

-   R (reflux ratio)=2.7

-   Column top temperature=134° C.

-   Pressure=10 mmHg, i.e. 1 333.2 Pa.

This column (I) is fed continuously with the flow rate of 18.9 kg/horiginating from the detarring.

A flow rate of approximately 2.45 kg/h is obtained at the column top,which flow rate comprises:

-   -   approximately 99.5% catechol    -   approximately 800 ppm hydroquinone    -   approximately 40 ppm resorcinol.

A flow rate of approximately 16.45 kg/h is obtained at the columnbottom, which flow rate comprises:

-   -   approximately 15.3 kg/h hydroquinone    -   approximately 1.05 kg/h resorcinol    -   approximately 180 ppm catechol    -   approximately 0.1 kg/h tars.        Distillation column (II):

-   n=30

-   R=10

-   Column top temperature: 170° C.

-   Pressure: 10 mmHg, i.e. 1 333.2 Pa.

It is fed continuously with the bottom product from the column (I) at aflow rate of approximately 16.45 kg/h.

A flow rate of approximately 0.75 kg/h of a resorcinol-rich fraction isobtained at the top, which fraction comprises:

-   -   approximately 0.65 kg/h resorcinol    -   approximately 0.1 kg/h hydroquinone    -   approximately 1 200 ppm catechol.

A flow rate of approximately 15.7 kg/h of a mixture is obtained at thebottom, which mixture comprises:

-   -   approximately 15.2 kg/h hydroquinone    -   approximately 0.4 kg/h resorcinol    -   approximately 0.1 kg/h tars.        (Distillation) detarring column (III):

-   Detarring column: scraped falling film device

-   Column top temperature: 174.5° C.

-   Pressure: 10 mmHg, i.e. 1 333.2 Pa.

This column is fed continuously with the bottom product from the column(II) at a flow rate of approximately 15.7 kg/h.

A flow rate of approximately 15.2 kg/h of a hydroquinone-rich fractionis obtained at the top, which fraction comprises:

-   -   approximately 14.8 kg/h hydroquinone    -   approximately 0.4 kg/h resorcinol.

A flow rate of approximately 0.5 kg/h of a mixture is obtained at thecolumn bottom, which mixture comprises:

-   -   approximately 0.4 kg/h hydroquinone approximately 0.1 kg/h tars.        Refining:

The refining is carried out as in example 1.

It must be clearly understood that the invention defined by the appendedclaims is not limited to the specific embodiments indicated in the abovedescription but encompasses the alternative forms thereof which departneither from the scope nor from the spirit of the present invention.

1. A process for separation and purification of a crude mixturecomprising hydroquinone and resorcinol, in which process the crudemixture is first subjected to a series of distillation stagescomprising: (i) subjecting the crude mixture to a distillation stage(II) designed to produce, as a distillation top product, aresorcinol-rich fraction comprising resorcinol, essentially, andhydroquinone; (ii) subjecting a distillation bottom product obtained bystep (i) to a distillation stage (III) designed to produce, as adistillation top product, a hydroquinone-rich fraction comprisinghydroquinone, essentially, and resorcinol; and then subjecting thehydroquinone-rich fraction and/or the resorcinol-rich fraction to arefining stage (IV or V) in order to extract the hydroquinone and/or theresorcinol, respectively, wherein the refining stage comprisessuccessive cooling/crystallization, draining of a eutectic, reheatingand draining a liquid fraction, and then melting-recovering acrystallized fraction.
 2. The process according to claim 1, wherein thehydroquinone-rich fraction obtained by distillation stage (III) issubmitted to a refining stage comprising the following successivephases: phase 1: slow cooling of the fraction for 5 to 15 h until atemperature of between 90 and 130° C. is reached; phase 2: draining theliquid fraction; phase 3: slow reheating for 8 to 24 h until atemperature of between 170 and 173° C. is reached, and draining theliquid fraction; and phase 4: heating to 178° C. and recoveringhydroquinone.
 3. The process according to 2, wherein, before phase 1,the hydroquinone-rich fraction is heated above the melting point ofhydroquinone.
 4. The process according to 3, wherein, before phase 1,the hydroquinone-rich fraction is heated to a temperature between 175and 180° C.
 5. The process according to claim 3 or 4, wherein phase 3 isstopped when the temperature reaches 171° C.
 6. The process according toany one of claims 1 to 4, wherein stages (II) and (III) are performed soas to obtain at the top of stage (III) a hydroquinone-rich fractioncomprising from 75 to 98% of hydroquinone and from 2 to 25% ofresorcinol, these percentages being expressed with respect to the sum ofhydroquinone and resorcinol.
 7. The process according to claim 6,wherein said hydroquinone-rich fraction comprises 85 to 97.5% ofhydroquinone and 2.5 to 15% of resorcinol.
 8. The process according toclaim 1, wherein the resorcinol-rich fraction obtained at the top ofstage (II) is submitted to a refining stage comprising the followingsuccessive phases: phase 1: slow cooling of the fraction for 5 to 15 huntil a temperature of between 40 and 90° C. is reached; phase 2:draining the liquid fraction; phase 3: slow reheating for 8 to 15 huntil a temperature of between 109 and 111° C. is reached, and drainingthe liquid fraction; and phase 4: heating to 115° C. and recoveringhydroquinone.
 9. The process according to claim 8, wherein, before phase1, the resorcinol-rich fraction is heated above the melting point ofresorcinol.
 10. The process according to claim 9, wherein, before phase1, the resorcinol-rich fraction is heated to a temperature between 115and 120° C.
 11. The process according to claim 8 or 10, wherein phase 3is stopped when the temperature reaches 111° C.
 12. The processaccording to any one of claims 8 to 10 wherein stage (III) is performedso as to obtain at the top of stage (II) a resorcinol-rich fractioncomprising from 75 to 98% of resorcinol and from 2 to 25% ofhydroquinone, these percentages being expressed with respect to the sumof hydroquinone and resorcinol.
 13. The process according to claim 12,wherein said resorcinol-rich fraction comprises 85 to 92% of resorcinoland 8 to 15% of hydroquinone.
 14. The process according to claim 1,wherein refining of the rich fractions is conducted on a drainer. 15.The process according to claim 1, wherein the crude mixture furthercomprises catechol and stage (II) is preceded by a distillation stage(I) designed to produce catechol as distillation top product, and thedistillation bottom product is used to feed stage (II).
 16. The processas claimed in claim 15, wherein stage (I), when it is present, or stage(II) is preceded by at least one predistillation stage (1,1′) designedto produce, as bottom product, a tar-rich fraction and, as top product,a detarred fraction which is used to feed stage (I) or stage (II). 17.The process as claimed in claim 16, wherein two predistillation stages(1,1′) are provided, the tar-rich bottom fraction from the firstpredistillation stage (1) being used to feed the second predistillationstage (1′) and wherein the two detarred top fractions are used to feedstage (I) or (II).
 18. The process as claimed in claim 15, wherein thedistillation column for distillation stage (I) has from 5 to 40theoretical stages; and a reflux ratio R of between 1 and
 10. 19. Theprocess as claimed in claim 18, wherein the distillation column has 10to 30 theoretical stages.
 20. The process as claimed in claim 18,wherein the reflux ratio R is between 2 and
 5. 21. The process asclaimed in claim 1, wherein the distillation column for distillationstage (II) has from 10 to 85 theoretical stages; and a reflux ratio R ofbetween 1 and
 35. 22. The process as claimed in claim 21, wherein thedistillation column has 15 to 40 theoretical stages.
 23. The process asclaimed in claim 21, wherein the reflux ratio R is between 5 and
 25. 24.The process as claimed in claim 1, wherein the distillation column fordistillation stage (III) is a scraped falling film device or adistillation column having from 1 to 10 theoretical stages; and a refluxratio R of between 0.5 and
 5. 25. The process as claimed in claim 24,wherein when the column is a distillation column, the column has 1 to 5theoretical stages.
 26. The process as claimed in claim 24, wherein whenthe column is a distillation column, the reflux ratio R is between 1 and2.
 27. The process as claimed in claim 16 or 17, wherein the column orcolumns for the predistillation stage (1,1′) is/are scraped falling filmdevices.
 28. The process as claimed in claim 1, wherein the crudemixture comprises, with respect to the total mixture: from 20 to 60% byweight of hydroquinone; from 2 to 20% by weight of resorcinol; from 0 to20% by weight of catechol; and the remainder being formed of variouscompounds, essentially tars.
 29. The process as claimed in claim 28,wherein the mixture comprises 30-50% by weight of hydroquinone.
 30. Theprocess as claimed in claim 28, wherein the mixture comprises 2 to 15%by weight of resorcinol.
 31. The process as claimed in claim 28, whereinthe mixture comprises 5 to 15% by weight of catechol.